(CNN) Dallas County Health and Human Services in Texas said it has received confirmation from the Centers for Disease Control and Prevention that a case of the mosquito-borne Zika virus was transmitted through sexual contact this year. The patient, the county said, “was infected with the virus after having sexual contact with an ill individual who returned from a country where Zika virus is present.” The county said the patient had recently traveled to to Venezuela, a country where Zika is actively transmitted, but the sexual partner had not traveled.

In a statement to CNN, the CDC said it confirmed the test results showing Zika present in the blood of a “nontraveler in the continental United States.” Based on that, the CDC has updated its advice: “The best way to avoid Zika virus infection is to prevent mosquito bites AND to avoid exposure to semen from someone who has been exposed to Zika virus or has been ill from Zika virus infection.” Earlier Tuesday, CDC Director Tom Friedan told CNN’s Chief Medical Correspondent Sanjay Gupta: “There have been isolated cases of spread through blood transfusion or sexual contact and that’s not very surprising. The virus is in the blood for about a week. How long it would remain in the semen is something that needs to be studied and we’re working on that now.”

Friedan added that studies on sexual transmission are not easy studies to do, but the CDC is continuing to explore that avenue of transmission. “What we know is the vast majority of spread is going to be from mosquitoes,” Freidan added. “The bottom line is mosquitoes are the real culprit here.” The CDC said it will provide more guidance as more information on sexual transmission is learned, but in the meantime, “Sexual partners can protect each other by using condoms to prevent spreading sexually transmitted infections. People who have Zika virus infection can protect others by preventing additional mosquito bites.”

History of Sexual Transmission

Before this case, there have been only two documented cases linking Zika to sex. During the 2013 Zika outbreak in French Polynesia, semen and urine samples from a 44-year-old Tahitian man tested positive for Zika even when blood samples did not. Five years before that, in 2008, a Colorado microbiologist named Brian Foy contracted Zika after traveling to Senegal; his wife came down with the disease a few days later even though she had not left northern Colorado and was not exposed to any mosquitoes carrying the virus.

In addition, the CDC said there have been documented cases of virus transmission during labor, blood transfusion, and laboratory exposure. While Zika has been found in breast milk, it’s not yet confirmed it can be passed to a baby through nursing.